The sensing means for a conveyor may consist of mechanical limit sensors or inductive transducers. The status data obtained from the sensing means of a conveyor provide combined information concerning the status of the conveyor, i.e. if the conveyor is free or if it is carrying a load (e.g. a bale), as well as the location of the conveyor. In this context “conveyor” shall also be understood to mean machines in the conveyor system other than true conveyors. From a communication point of view, all such machines must function as conveyors.
Conveyor equipment in the pulp baling line for the manufacture of board, for instance, is seldom the same from case to case. This is primarily because the premises in which the conveyor equipment is to be installed are seldom similar, but also because the form of the load (bale) and the capacity required for the conveyor system differs from plant to plant. Each plant is specifically designed, thus requiring extensive engineering work upon delivery. In a typical case approximately half the engineering work is devoted to the conveyors and half to the other machinery included in the plant.
Control systems in modern conveyor systems of the type under discussion are also constructed so that the control unit for each conveyor is arranged in a central computer common to the conveyor system. The central computer thus includes a control unit for each conveyor so that each control unit is in reality a program module. Each conveyor communicates through its control unit with all other conveyors on which the conveyor in question may be dependent. The control unit receives status data from the conveyors on which it is dependent. A conveyor is dependent on the conveyors whose status affects how the conveyor in question shall be controlled. Purely theoretically a conveyor may be dependent on an infinite number of other conveyors. In practice, however, a conveyor is usually dependent on not more than about 15 other conveyors and its control unit thus receives status data from 15 other conveyors. On the basis of the status data the control unit creates control signals to control the conveyor pertaining to the control unit.
Since each conveyor in a conveyor system has its own set of conveyors on which it is dependent, and since two conveyor systems are hardly ever alike, extremely complex, customer-unique solutions are required for each conveyor system. In principle, therefore, a unique control unit is required for each conveyor.
If for some reason the plant is to be altered, by a conveyor being removed, for instance, or being given a slightly altered function, the control unit of this conveyor and the control units of all conveyors dependent on this conveyor must then be reprogrammed.
An object of the present invention is to avoid these problems and thereby, amongst other things, to greatly reduce the engineering work, and consequently the costs involved.